Spark-ignition engines operate with a homogeneous fuel-air mixture which, in the absence of direct injection, is prepared through external mixture formation by introducing fuel into the intake air in the intake tract. The required power output is adjusted by varying the combustion chamber charge so that, unlike in the diesel engine, operation of the spark-ignition engine is based on a quantitative control.
The load is generally controlled by means of a throttle valve provided in the intake tract. By adjusting the throttle valve the pressure of the intake air downstream of the throttle valve can be reduced to a greater or lesser degree. The further the throttle valve is closed, the greater the pressure loss of the intake air over the throttle valve and the lower the pressure of the intake air downstream of the throttle valve and upstream of the inlet to the cylinder. Given a constant combustion chamber volume it is possible in this way to adjust the air mass, that is to say the quantity, by way of the intake air pressure. However, in the partial load range, since small loads require a high degree of throttling and pressure reduction in the intake tract, charge cycle losses increase as the load diminishes and the throttling increases. As a result, engine efficiency and thus fuel economy are compromised.
Various strategies have been developed for dethrottling an internal combustion engine with applied ignition, in order to reduce the losses described. Since in partial load operation the spark-ignition engine has a poor efficiency due to the throttle control, whereas the diesel engine has a greater efficiency, attempts have been made to combine the two methods of operation with one another, in order to exploit the advantages of the diesel engine method for the benefit of the spark-ignition engine method. Here the development work has concentrated primarily on the essential features of the two methods. The conventional spark-ignition method is characterized by a mixture compression, a homogeneous mixture, an applied ignition, and the quantitative control, whereas the conventional diesel engine method is characterized by an air compression, an inhomogeneous mixture, a compression ignition and the qualitative control.
One approach to dethrottling, for example, is to operate the spark-ignition engine with direct injection. Direct fuel injection is a suitable means for achieving a stratified combustion chamber charge. Within certain limits, the direct injection of fuel into the combustion chamber thereby allows a qualitative control in the spark-ignition engine. The mixture formation ensues through the direct injection of fuel into the cylinders or rather the air present in the cylinders and not through external mixture formation, in which the fuel is introduced into the intake air in the intake tract.
Another possible way of optimizing the combustion process of a spark-ignition engine lies in the use of a variable valve gear. In contrast to conventional valve gears, in which both the valve lift and also the timings, that is to say the opening and closing times of the intake and exhaust valves, are predetermined as invariable quantities by the non-adjustable and hence inflexible mechanism of the valve gear, these parameters influencing the combustion process and thereby the fuel consumption can be varied to a greater or lesser degree by means of variable valve gears. A load control with no throttle and thereby no losses is possible simply by being able to vary the closing time of the intake valve and the intake valve lift.
The concepts described above have the disadvantage that they are not suitable for retrofitting to engines already on the market, since they require substantial modifications to the basic engine and/or the valve gear, and additional complex components.
One approach to the dethrottling of spark-ignition engines already on the market is afforded by the cylinder cutoff. This serves to improve, that is to say to increase the efficiency in the partial-load range since the cutoff of one cylinder of a multi-cylinder internal combustion engine increases the load of the cylinders in operation, so that the throttle valve may be opened further in order to introduce a larger air mass into these cylinders, so that overall a dethrottling of the internal combustion engine is achieved. Owing to the larger air mass delivered, the cylinders still being operated during the partial cutoff have an improved mixture formation and tolerate higher exhaust gas recirculation rates. Further advantages in terms of efficiency accrue in that owing to the absence of combustion a cylinder which has been cut off does not generate any heat losses through the wall due the transmission of heat from the combustion gases to the combustion chamber walls.
Besides the aforementioned advantages, partial cutoff, particularly in multi-cylinder internal combustion engines having an odd number n of cylinders, also have disadvantages, which are often an obstacle to use in series production. Conventionally, in an inline three-cylinder engine, for example, one cylinder of the engine is embodied as a cutoff cylinder. In normal operation, that is to say when all three cylinders are in operation and the partial cutoff is deactivated, the cylinders are fired in the firing order 1-2-3 at an interval of 240° CA. In the context of a partial cutoff, the cutoff cylinder is deactivated and only the two remaining cylinders continue to operate, so that an irregular firing pattern ensues, in which the firing interval alternates between 240° CA and 480° CA, which results in several detrimental effects.
The engine structure excited to structure-borne sound oscillations by the impulses and alternating forces emits the structure-borne sound via its engine surfaces as airborne sound and in this way generates the actual engine noise. The irregular firing pattern leads to an unharmonious engine noise, which is perceived as unpleasant. This is disadvantageous, since the noise generated by the internal combustion engine has a considerable influence on customers' purchasing behavior. Further, excitation of the crankshaft in the natural frequency range can result in high rotational oscillation amplitudes, which can even lead to fatigue fracture.
The problems discussed taking a three-cylinder internal combustion engine as an example similarly exist in any multi-cylinder internal combustion engine, in which an odd number n of cylinders is arranged in line, for example also in the case of a five-cylinder internal combustion engine, in which five cylinders are arranged in line.
The inventors herein have recognized the above issues and provide a solution to at least partially address them. Thus, a method for operating a multi-cylinder internal combustion engine with applied ignition having an odd number n of cylinders arranged in line is provided. The method comprises operating a multi-cylinder internal combustion engine with applied ignition, in which an odd number n of cylinders is arranged in line, and, during partial-load operation when engine load is below threshold, enabling a partial cutoff of the cylinders, the partial cutoff comprising operating each cylinder only intermittently such that each cylinder is fired and cut off in turn at an interval of (2*720° CA)/n.
In the method according to the present disclosure, in normal operation, when all n cylinders are being operated and the partial cutoff is deactivated, the n cylinders are fired at a firing interval of approximately 720° CA/n. During the partial cutoff, on the other hand, each cylinder is operated intermittently and in such a way that each cylinder is fired and cut off in turn, so that in partial load operation the cylinders are fired in a modified firing order and at a firing interval of approximately (2*720° CA)/(n). The firing interval therefore doubles with a partial cutoff of the cylinders. The partial cutoff according to the disclosure leads to a uniform firing interval, that is to say to a regular firing pattern, and thereby to a harmonious engine noise.
In a multi-cylinder, in-line engine having an odd number of cylinders and applied ignition, the method for partial cutoff according to the present disclosure makes it possible to reduce the charge cycle losses which are bound to occur due to the quantitative control by means of a throttle valve, whilst avoiding an irregular firing pattern, in which the firing interval varies and which has a detrimental effect on the noise emissions. Thus, dethrottling of the engine can be achieved without having to accept disadvantages in terms of the noise emissions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.